Orthogonal frequency division multiplexing (OFDM) is a multi-carrier transmission technique in which a user transmits on many orthogonal frequencies (or subcarriers). The orthogonal subcarriers are individually modulated and separated in frequency such that they do not interfere with one another. This provides high spectral efficiency and resistance to multipath effects. An orthogonal frequency division multiple access (OFDMA) system allows some subcarriers to be assigned to different users, rather than to a single user. Today, OFDM and OFDMA techniques are used in both wireline transmission systems and wireless transmission systems.
In conventional OFDM/OFDMA networks, a dedicated resource is allocated to each subscriber station (e.g., mobile device, wireless terminal, etc.) for ARQ feedback or hybrid ARQ feedback, such as an acknowledgment (ACK) message or a negative acknowledgment (NACK) message. By way of example, a transmitter (e.g., base station) in an OFDM network sends data packets and control information to a receiver (e.g., subscriber station). The control channel carries information specifying, for example, the sequence number and modulation and coding scheme used to encode the packets. The subscriber station tries to decode the packets and transmits to the base station in the ACK/NACK channel a feedback message regarding a successful or an unsuccessful transmission.
In conventional systems, the ACK/NACK signal is time-multiplexed within a subframe of the uplink or reverse channel (i.e., from subscriber station to base station) using a very short transmission duration. However, an ACK/NACK signal transmitted for only a short duration carries only a small amount of energy. Also, since a conventional network uses dedicated resources (i.e., time-frequency slots) to transmit ACK/NACK messages, the ACK/NACK channels are allocated without considering the instantaneous channel quality of the ACK/NACK channels. This results in very high bit error rate (BER) in the ACK/NACK channel, thereby resulting in limited system throughput and coverage.
Therefore, there is a need for improved OFDM/OFDMA systems that maximize throughput and coverage. In particular, there is a need for improved OFDM/OFDMA systems that transmit ACK/NACK signals in the uplink with a lower bit error rate.